A popular view is that we are made up of three parts (trichotomy), not two. We have two immaterial components—both a soul and a spirit. Usually, these are given different meanings (e.g., "spirit" is our life-principle, and "soul" is the part of a person that relates to God). As seen in the previous discussion, however, these distinctions are not maintained by a comprehensive examination of Scripture. The only verses which seem to support trichotomy are 1 Thess. 5:23 and Heb. 4:12. Read 1 Thess. 5:23, and then apply some hermeneutical principles to it. Is Paul interested in defining the way humans are made up in this context? What is his real concern? If he’s not interested in defining personhood here, and could be using an imprecise expression which connotes wholeness, maybe it would be unwise to take this one verse and make a whole theology out of it—especially when Paul elsewhere uses the terms "spirit" and "soul" interchangeably.
Hebrews 4:12 is considered a secondary support for trichotomy, since the author of Hebrews talks about the "division" of soul and spirit, and this is interpreted to mean that soul and spirit are different things. Read this verse also, and ask some hermeneutical questions of it. Is the language used here literal or figurative? What is the author’s real point? If it’s a poetic and figurative passage, is it wise to build a doctrine out of the language?
Consider the use of balanced terms throughout the verse: living/active, soul/spirit, joints/marrow, thoughts/intentions. Note that these are not meant to be strict opposites in all the other cases; in fact, these are collections of synonyms, used in parallel for poetic emphasis. Also I would point out that the "division" of soul and spirit does not mean they are divided one from another. It means that our lives are penetrated (divided, opened up) by God’s word to the depth of our spiritual selves.